The present invention relates to a contact printer which is mainly used for plate making, and more particularly relates to a printing frame of a contact printer, on which an original plate and a photosensitive material are mounted one upon another tightly on a transparent plate.
In a conventional contact printer, when the printing is carried out, an original plate and a photosensitive material are laid one upon another on a transparent plate of a printing frame, and are covered by a flexible airtight sheet made of a flexible material such as rubber and plastic material, under vacuum, so as to contact the original plate and the photosensitive material onto the transparent plate one upon another tightly. Then, the contact printing is performed by exposing the light to the transparent plate.
In a conventional semi-automatic contact printer which performs such an operation, the flexible airtight sheet is wound round a spring roller which is biased by a coil spring arranged therein to rotate on its own axis and to roll over the transparent plate, and the free end of the flexible airtight sheet is fixed to the front end of the transparent plate. Hence, the airtight sheet can be extended to cover the original plate and the photosensitive material laid on the transparent plate one upon another, by rolling the spring roller over the transparent plate towards its rear end against the biasing force of the coil spring.
Further, a squeeze roller is arranged before the spring roller and rolls on the airtight sheet together with the spring roller to squeeze air from the space between the airtight sheet and the transparent plate, and then the air in the space between the airtight sheet and the transparent plate is sucked under vacuum, for example, by a vacuum pump.
However, in the conventional printer, since the height of the spring roller rolling over the transparent plate is maintained at the predetermined level, the diameter of the airtight sheet wound on the spring roller is reduced, as the spring roller is rolled over the transparent plate towards its rear end. Thus, the inclined contact angle of the airtight sheet with respect to the transparent plate increases gradually, and the tension of the airtight sheet due to the biasing force of the coil spring also increases gradually, as the spring roller is rolled over the transparent plate rearwards. Accordingly, the squeeze roller is liable to float, and hence it threatens to perform insufficient squeezing of the air from the space between the airtight sheet and the transparent plate.
Further, in the conventional printer, since the spring roller and the squeeze roller are placed on the surface of the transparent plate, it often obstructs the operation during the mount and the demount of the original plate and the photosensitive material, and spoils the appearance.